Funds are requested to carry out NMR studies to develop a deeper understanding of the dynamic behavior of G-Protein Coupled Receptors (GPCRs) and their signaling complexes to enable the construction of a better model of their mechanism of action. GPCRs are critical eukaryotic signal transduction gatekeepers which recognize a variety of extracellular stimuli, including photons, ions, small molecules, peptides and proteins. They transmit the resulting extracellular signal across the membrane by coupling to different intracellular proteins which then activate downstream effectors and trigger cascades of cellular and physiological responses. This has profound therapeutic implications, making these receptors the targets of ~30% of currently prescribed drugs. Developing a better understanding of how they work is, therefore, expected to help in developing new and better drugs targeting them. Although there has been a dramatic growth in the number of structures of GPCRs bound to antagonists, agonists including biased ligands, inverse agonists, and allosteric modulators, the molecular mechanisms of activation remain poorly understood. The proposed study rests on the premise that needed clarity can be achieved with a better understanding of the dynamic behavior of the system. We plan to characterize the dynamic behavior of all the members of the opioid family of receptors (Class A, ?-subfamily). There are four members in this family - the classical opioid receptors (OR) ?-, ?-, ?, (MOR, KOR, DOR) and the nociceptn receptor (NOP), all of which are coupled predominantly to heterotrimeric Gi/Go proteins. ORs are acted on by opioids which have had a long history in both chronic and acute pain control and addiction. This funding request is to support studies on KOR and the development of new knowledge and protocols that could be applied to the rest of the family. We will achieve a better understanding of the mechanism of action including the activation process through three specific aims: (1) Develop an understanding of the dynamic conformational landscape of KOR in the active and inactive state, (2) Develop an understanding of the dynamic behavior of the KOR signaling complex, and (3) Develop an understanding of the KOR-dynorphin interaction in the low and high affinity states. In this proposal, we will use our newly developed 19F NMR approach to characterize the dynamic behavior of KOR and standard NMR techniques for 2H/13C/15N NMR studies of the peptide dynorphin and the signaling partners to characterize changes to their dynamic behavior on binding to KOR. Studies will allow us to explore various questions, answers to which will help us build a better model describing the activation process. In addition, protocols and reagents generated in the study should help establish a new platform for developing new drugs targeting opioid receptors with reduced or minimal side-effects.